This invention relates to an anti-tamper assembly for a circuit board comprising one or more electronic components, the assembly comprising: a container having side walls, a first, closed end and a second, opposing, open end, the container being configured to be mounted on said circuit board at said open end, over at least one of said electrical components, to form, in use, a sealed cavity around said at least one of said electrical components; a source of radioactive particles mounted within said container; an image sensor for capturing image frames within said sealed cavity, in use, wherein said image sensor comprises a detector region defining an array of pixels; and a processor for receiving said captured image frames, monitoring said image frames for changes in the statistical distribution of active pixels and, in the event that statistical distribution of active pixels indicates the presence of a feature in an image frame, generating a tamper alert.